(a) Field of the Invention
The invention relates generally to the commercial refrigeration art, and more particularly to fluid defrost system and method improvements in secondary refrigeration systems for cooling food product merchandisers or the like.
(b) Related Cases
This application discloses improvement subject matter related to (1) co-pending and commonly-owned application Ser. No. 08/631,104 filed Apr. 12, 1996 U.S. Pat. No. 5,727,393 for Multi-Stage Cooling System for Commercial Refrigeration (Mahmoudzadeh), and (2) co-pending and commonly-owned application Ser. No. 08/632,219 filed Apr. 15, 1996 U.S. Pat. No. 5,743,102 for Strategic Modular Secondary Refrigeration (Thomas et al).
(c) Description of the Prior Art
World-wide environmental concerns over the depletion of the protective ozone layer and resultant earth warming due to releases of various CFC (chlorofluorocarbon) base chemicals into the atmosphere has resulted in national and international laws and regulations for the elimination and/or reduction in the production and use of such CFC chemicals. The refrigeration industry in general has been a primary target for government regulation with the result that some refrigerants, such as R-502, previously in common use in commercial foodstore refrigeration for many years are now being replaced by newer non-CFC types of refrigerants. However, such newer refrigerants are even more expensive than the more conventional CFC types, thereby raising basic cooling system installation and maintenance costs and creating higher loss risks in conventional backroom types of commercial systems having long refrigerant piping lines from the machine room to the store merchandisers. For instance, in a typical large supermarket of 50,000 square feet, the aggregate refrigeration capacity of the various food merchandisers, coolers and preparation rooms may exceed 80 tons (1,000,000 BTU/hr.) including 20 tons of low temperature refrigeration and 60 tons of medium temperature refrigeration. In this example, the piping length would be on the order of 18,000 feet of conduit requiring about 1800 pounds of refrigerant. One of the newer refrigerants is R-404A (an HFC chemical) that now costs about $8.00 per pound.
Obviously, the refrigeration industry has been is concerned over its role in the environmental crisis, and has been seeking new refrigeration systems and applications for non-CFC chemicals in attempting to help control the CFC problem while maintaining high efficiency in food preservation technology.
So-called "cascade" or staged refrigeration systems are well-known, especially where relatively low temperatures are required in controlled zones such as in industrial refrigeration and cryogenic applications. Commonly-owned U.S. Pat. No. 5,440,894 discloses improvements in commercial foodstore refrigeration systems utilizing modular first stage closed-loop refrigeration units of the vapor compression type that are strategically located throughout the foodstore shopping arena in close proximity to groups of temperature-associated merchandisers (i.e. "close coupled"), and preferably having an efficient condenser heat exchange network through a cascade-type coolant circulating system. This prior cascade-type system is representative of a typical "two fluid" approach to multi-stage refrigeration in that the mechanical vapor-compression refrigeration stage is still the final, direct refrigeration step in the controlled cooling of the merchandiser evaporator coils for maintaining product zone temperatures, and the other liquid or fluid coolant is circulated in cooling heat exchange with the refrigeration system condensers. Commonly-owned U.S. patent application Nos. 08/631,104 U.S. Pat. No. 5,227,393 and 08/632,219 U.S. Pat. No. 5,743,102 (previously cited) also disclose cascade-type "two fluid" systems, now more commonly called "secondary refrigeration systems" in which the vapor compression central system cools a secondary non-compressible coolant fluid, such as propylene glycol solutions, for direct distribution to the cooling coils of product display fixtures or the like. Other prior art references of the "two fluid" type include the following patents:
______________________________________ U.S. Patents Date Inventor ______________________________________ 3,210,957 10/1965 Rutishauser 3,675,441 07/1972 Perez 4,280,335 07/1981 Perez et al 4,344,296 08/1982 Staples et al 5,335,508 08/1994 Tippmann ______________________________________
EPO publication No. 0483161 B1 published Jun. 29, 1994 discloses another multi-stage refrigeration system in which a central, vapor-compression, refrigeration unit cools a "secondary" coolant fluid circulated for the direct primary cooling of a medium temperature unit and thence in series flow for cooling the condenser of a self-contained fixture.
In any commercial system to maintain the product zone temperatures for frozen foods, fresh meat and dairy products or other refrigerated products, it is known that the cooling (evaporator) coils or heat exchangers for such product zones must be maintained at or below the freezing point of water with a resultant frost or ice build-up during cooling operations. In order to maintain the heat transfer efficiency of such heat exchangers to cool circulating air flow to the product zone and minimize unwanted temperature rise in the product area, periodic defrosting of the heat exchangers must be performed as expeditiously as possible. Conventional forms of defrosting the evaporator coils in low and medium temperature vapor-compression systems include electric, hot gas and saturated gas defrosting and some off-cycle defrosting in higher temperature systems. The use of hot gas from the compressor discharge is widely used in refrigeration, and utilizing saturated gas from the receiver (as taught by Quick U.S. Pat. No. 3,343,375) is also known in the industry. The secondary refrigeration systems of co-pending U.S. patent application Nos. 08/631,104 U.S. Pat. No. 5,727,393 and 08/632,219 U.S. Pat. No. 5,743,102 disclose the use of hot coolant, similar to hot gas, for low and medium temperature system operations, but over-heating problems have been encountered.